Patent GB-701852-A (GB)
Title: Improvements connected with remote or automatic control systems for dynamo-electric machines
IPC: H02P23/00
Filed: 1951-10-01
Abstract: 701,852. Controllers, control of A.C. motors. LAURENCE, SCOTT & ELECTROMOTORS, Ltd., and SCHWARZ, K. K. Sept. 30, 1952 [Oct. 1, 1951], No. 22842/51. Class 38(3) [Also in Group XXXVIII] A control system for .dynamo-electric machines comprises two similar relays, R1, R2, having operating coils 1, 2, forming two adjacent arms of a bridge circuit the two other arms of which consist of impedances 3, 4. The bridge circuit is supplied at a voltage of approximately twice the operating voltage of the relays, and control voltages Vp, Vs, are differentially applied between points X and Y, Vp being a pre-set voltage and Vs a signal voltage. Both relays are operated when the resultant voltage between the points X, Y, is zero and one relay only is operated in accordance with the direction of the resultant control voltage. Thus, when the voltage Vp exceeds the voltage Vs, the voltage applied to the relay R1 will be greater than that applied to the relay R2, and, if the relay coils are initially unexcited, the relay R1 will operate whereas the relay R2 will remain open. As shown, each of the four control circuits r-v are closed in turn according to the conditions of the control relays. Since the relays when once closed will not drop out until the applied voltage has fallen to a low value, resetting means must be provided. Such means may comprise a push-button for interrupting the supply to the relays and placed in circuit between the operating coils of the relays, Fig. 1A (not shown), or a switch coupled to a potentiometer for presetting the voltage Vp. Automatic re-setting of the main relays Rl, R2, may be obtained by a discriminating relay connected across the centre of the bridge circuit so that its coil is subjected to the difference of the voltage Vp, Vs. The discriminating relay has contacts for interrupting the supply to the main relays, and the main relays have contacts for the re-setting and protection of the discriminating relay, Fig. 2 (not shown). Thus when the difference between the voltages Vp, Vs, exceeds a pre-determined value, the discriminating relay momentarily releases the relays Rl, R2, which have contacts for de-energizing the discriminating relay. The main relays then set in operation a follow-up system for correcting the voltage-difference and when the balanced condition is reached the discriminating relay is re-connected. The control voltage Vs only may be applied across the centre of the bridge, the voltage Vp being obtained by a movable tapping on a single resistor which replaces resistors 3, 4, Fig. 3 (not shown). The system may be applied to the speed setting of a three phase stator fed A.C. commutator motor controlled by an induction regulator adjusted by a pilot motor, Fig. 4 (not shown). The rectified signal voltage Vs is obtained from the rotor of the commutator motor and is boosted by a transformer to prevent it from passing through zero. The control voltage Vp is obtained from a potentiometer supplied from a transformer through a rectifier. The pilot motor for operating the induction regulator is of the three phase squirrel cage type, and the relays R1, R2, are adapted to control the pilot motor by means of two change-over contacts on each control relay so that the pilot motor is reversed by the opening of one relay and the closing of the other.